The invention relates to a method for determining the changing location of the point incidence of a continuous or pulsed energetic beam on a delimited surface, which is moved periodically over the surface by a deflecting unit, wherein the surface is detected with a camera having a plurality of individually evaluable pixels in an image plane, wherein each point of the surface is associated with a pixel and wherein the image values detected by the camera are evaluated by means of an image analysis device.
US 2005/205778 describes a system for calibrating the deflection of a laser beam to remove material from electronic components in order to modify their properties. A test card with a defined pattern is used to detect imaging errors, which allows the detected imaging errors to be compensated for.
The energetic beam may be, for example, an electron beam generated by an electron gun, which is directed at the melt surface to evaporate material there, which serves for the coating of components. Other uses of energetic beams are discussed below. In order to obtain a uniform energy input into the surface, a deflecting unit is provided which moves the energetic beam periodically in a predetermined pattern over the surface. The beam can be operated continuously or pulsed, wherein a new point of incidence is defined for each pulse, so that a periodically recurring pattern of points of incidence results.
Since the position of the surface to be detected with respect to the electron beam gun is known, the deflecting unit can be adjusted so that the pattern covers the entire delimited surface.
However, it has become apparent that deviations from the set target pattern result due to external influences. Since the effects of the influences on the beam are not necessarily reproducible, they cannot be considered in advance in a corresponding control function for the deflection. Therefore, it is necessary to provide the operator of a corresponding apparatus with an image of the surface on which the migrating point of incidence can be seen, or the pattern that arises. If the operator recognizes that the path of the point of incidence deviates from the desired path or the pattern deviates from the set target pattern, he/she can readjust the deflecting unit accordingly.
DE 10 2008 009 410 B4 already discloses the automation of this process. For this purpose, an analysis is made of the image of the surface scanned by the beam and, with the aid of image analysis, the path of the point of incidence is compared with the desired path, if deviations occur, a corresponding automatic readjustment of the deflecting unit takes place.
Such automatic image analysis is difficult, however, because, for example, the melt surface is very bright, so that the point of incidence, although it should have a higher brightness than its surroundings due to a corresponding energy input, is difficult to see because its brightness differs only slightly from the brightness of the background.
The prior art also includes DE 10 2013 107 454 A1, which also deals with delimiting the movement of an electron beam over a surface on a certain area, e.g., a melt surface. For this purpose, a sensor is used, which registers the reactions of the incidences of the electron beam on the scanned surface. These reactions can be electromagnetic radiation or secondary electrons. However, the sensor is not a camera having a plurality of pixels each associated with surface points. The sensor does not record the electromagnetic radiation or the secondary electrons rasterized into pixels, but as a single value, which averages the reaction over the entire scanned surface.
It should also be taken into account that processes in which energetic, in particular high-energy beams, scan a surface take place in an encapsulated device. However, a camera for image analysis is placed outside the device and receives the image of the surface through a transparent disk, which reduces the image intensity in an unpredictable manner due to deposits of vapors that arise when scanning the surface, so that brightness differences in the image can no longer be assigned with certainty to a point of incidence.
In this context, when talking about a camera, it can, in principle, be a camera that picks up light in the visible wavelength range, infrared or some other frequency range. What is also meant are image acquisition systems that are based on other physical principles, as explained in more detail below. However, what is decisive is that the surface is imaged onto an image plane in the image acquisition system and that the image plane pixels are detected individually, so that they can be evaluated individually.
Preferably, however, the camera is a CCD camera with two-dimensional CCD array sensors, each individual CCD sensor of the arrays representing one pixel.
While a trained operator of a corresponding apparatus will still be able to determine the path of the point of incidence even in a poor image and manually make corresponding corrections on the deflecting unit, it is necessary—in particular for an automatic control—to perform especially reliable image recognition, in which the path of the point of incidence and the pattern thus generated can be reliably identified.
The invention is therefore based on the task of creating an image analysis in which the path of the point of incidence of an energetic beam on a delimited surface can be reliably identified.
For this purpose, the invention provides that:
the image analysis device determines a temporal sequence of image values for at least some of the pixels,
a signal analysis of the temporal sequence of image values of these pixels is made,
the results of the signal analysis are correlated with the periodicity induced by the deflecting unit and/or the periodicity of the beam pulse,
and, based on the quality of the correlation, it is determined whether the point of the surface associated with the respective pixel has been hit by the energetic beam at a particular time.
Since the continuous or pulsed beam is moved periodically by the control unit over the delimited surface, this periodicity is also found in the temporal sequence of image values for a pixel. This means that for each pixel a specific image value variation results, which is determined by the periodically recurring incidence of the beam on the surface point. If one now considers the temporal sequence of the image values for a pixel, one can determine its frequency spectrum and filter out the period frequency. In this way, individual bright image values, which are, however, caused by other than the incidence of the beam, are suppressed in the analysis.
It can therefore be determined by suitable mathematical methods whether the sequence of image values is characteristic for the incidence of the beam, so that, when an analysis is carried out for corresponding further pixels, a temporal course of the migration of the point of incidence on the surface can be determined. This can be done both for continuous beams and for pulsed beams, wherein in the latter case the pulse frequency can be included in the frequency analysis of the temporal sequence of image values.
Preferably, the continuous or pulsed beam is moved over the surface by a deflecting unit in a pattern that repeats periodically at a base frequency. In such a case, this base frequency can be used for the analysis of the sequence of image values.
For this purpose, for example, a spectrum of the temporal sequence can be determined, then the repetition frequency of the pattern filtered out and transformed back into a temporal sequence.
In addition to the base frequency, which describes the repetition of the pattern, there are other frequencies in the image, namely the oscillation frequencies which result from the frequency spectra with which the beam is deflected in the two planar directions, which are usually perpendicular to one another. These frequency spectra can also be used to analyze the sequence of image values.
The invention therefore provides that the beam is deflected by a deflecting unit for each of the two planar directions with a respectively specific frequency spectrum, the results of the signal analysis being correlated with the specific frequency spectra.
Another frequency, which can be used to analyze the sequence of image values, when using a pulsed beam, is its pulse rate.
As mentioned above, the beam is typically an electron beam used to re-melt or vaporize material. In this respect, the delimited surface is the surface of a melt.
However, observations of X-ray, laser or particle beams would also be conceivable. Laser beams are, for example, also used in 3D printers. Again, there are at least partially periodically recurring patterns on straight or curved surfaces, so that the invention is also suitable for controlling a three-dimensional print.
As mentioned above, with the incidence of the beam, certain physical processes are induced on the material forming the surface. Thus, in a melt, in addition to visible light, X-rays and particle beams which emanate from the point of incidence are generated. These can also be used to display an image using a corresponding image acquisition system.
As mentioned above, the method does not serve only to display an image on which the path of the point of incidence over the surface is visible, thus allowing an operator to make appropriate adjustments to a deflecting unit. Rather, the method can also be supplemented so that the information obtained on the path of the energetic beam over the surface flows into the control of the deflecting unit as a control variable.
Since, as explained, the prescribed method for image recognition permits reliable identification of the path of the deflection beam, a corresponding control algorithm for controlling the deflecting unit, which does not lead to a maladjustment, can also be set up.
An apparatus for carrying out the method preferably consists of a crucible for receiving a melt, an electron beam gun, which is directed onto the crucible, a deflecting unit controllable by means of a control device which moves the electron beam of the electron beam gun periodically over the melt surface, a camera, which is directed into the crucible and has a plurality of individually evaluable pixels in an image plane, each point of the surface being assigned a pixel, and with an image analysis device, characterized in that the image analysis device is arranged so that the periodicity of the beam deflection is used for image analysis to identify the points of incidence in the image.
The device may be supplemented in the sense that the image analysis device is adapted to output a correction value to the control of the deflecting unit.
In the following, the invention will be explained in more detail with reference to an embodiment.